Magnetic discs and disc drives provide quick access to vast amounts of stored information. Both flexible and rigid discs are available. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments.
FIG. 1 shows the schematic arrangement of a magnetic disk drive 10 using a rotary actuator. A disk or medium 11 is mounted on a spindle 12 and rotated at a predetermined speed. The rotary actuator comprises an arm 15 to which is coupled a suspension 14. A magnetic head 13 is mounted at the distal end of the suspension 14. The magnetic head 13 is brought into near-contact (<1 micro-inch separation) with the recording/reproduction surface of the disk 11. A voice coil motor 19 as a kind of linear motor is provided to the other end of the arm 15. The arm 15 is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion 17.
FIG. 2 shows a cross sectional view of a longitudinal recording disk medium. A longitudinal recording medium typically comprises a non-magnetic substrate 20 having sequentially deposited on each side thereof; an underlayer 21, 21′, such as chromium (Cr) or a Cr-containing alloy; a magnetic layer 22, 22′, typically comprising a cobalt (Co)-containing alloy; and a protective overcoat 23, 23′, typically containing carbon. General practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 21, 21′, magnetic layer 22, 22′, and protective overcoat 23, 23′, are typically deposited by sputtering techniques. The Co-containing alloy magnetic layer deposited by sputtering techniques normally comprises polycrystallites epitaxially grown on the polycrystalline Cr or Cr-containing underlayer.
A recording disk medium is prepared by depositing multiple layers of films to make a composite film. In sequential order, the multiple layers typically comprise a non-magnetic substrate, one or more underlayers, one or more magnetic layers, and a protective carbon layer.
Methods for manufacturing a longitudinal magnetic recording medium generally include a polycrystalline epitaxially grown cobalt-chromium (CoCr) alloy magnetic layer deposited on a chromium or chromium-alloy underlayer. Manufacturing longitudinal media with a glass, glass-ceramic, Al or Al-NiP substrate may also comprise applying a seed layer between the substrate and underlayer. A seed layer seeds the nucleation of a particular crystallographic texture of the underlayer. A seed layer is the first deposited layer on the non-magnetic substrate. The role of this layer is to texture (align) the crystallographic orientation of the subsequent Cr-containing underlayer.
Conventionally, the seed layer, underlayer, and magnetic layer are sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of argon. A carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Lubricant topcoats are typically less than 20 Å thick.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Al-Mg) alloy. Such Al-Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 10 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials such as glass, glass-ceramic material that comprises a mixture of amorphous and crystalline materials, and ceramic materials have been employed. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
Longitudinal magnetic recording media having Cr<200> and Co<11.0> crystallographic preferred growth orientations (hereafter orientations) are usually referred as bi-crystal media, and are commonly used in the industry. Here, Cr<200> refers to a body centered cubic (bcc) structured Cr-containing alloy underlayer or a B2-structured underlayer, either having a <200> preferred growth orientation. Typical bi-crystal media comprise Cr-containing alloy underlayers and Co-containing alloy magnetic layers. Uni-crystal media, which have a Co<10.0> preferred orientation; and randomly oriented media have also been used. Perpendicular magnetic recording media having Co<0002> preferred orientation are also being used. All of these media types typically have at least one small grain, hcp Co-containing alloy magnetic layer with low exchange coupling.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc); remanent coercivity (Hr); magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material; coercivity squareness (S*); signal-to-medium noise ratio (SMNR); track recording performance and thermal stability. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media. For example, decreasing the grain size or reducing exchange coupling between grains, can increase SMNR, but it has been observed that the thermal stability of the media often decreases.
As the storage density of magnetic recording disks has increased, the product of Mr and the magnetic layer thickness (t) has decreased and Hr of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hr. To achieve a reduction in Mrt, the thickness t of the magnetic layer has been reduced, but only to a limit because the magnetization in the layer becomes susceptible to thermal decay.
Medium noise in thin films is a dominant factor restricting increased recording density of high-density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or Co alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (hcp) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis that lies in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance and coercive force. For very small grain sizes, coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains, thereby decreasing intergranular exchange coupling. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, and properly using underlayers and seedlayers.
It is recognized that the magnetic properties, such as Hcr, Mr, S and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer, which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particularly for growing fine grains of hcp Co alloys deposited thereon.
For high signal to noise ratio (SNR) magnetic recording media, it is desirable to have a high signal in a very thin film. Higher signal can be achieved by increasing the saturation magnetization (Ms) of the material at the top of the magnetic layer, and correspondingly increasing the fringing magnetic field that provides signal. Prior art magnetic recording systems generally employ media including a magnetic layer alloy including Co and Cr, and other elements often including Pt, and B. These magnetic layer systems generally require 10-25% Cr, and often use 5-15% B in order to isolate the magnetic grains in the magnetic layer and reduce noise. Unfortunately, some of the Cr and B remains in the magnetic grains, reducing Ms to below 500 emu/cc, and correspondingly reducing the signal output of the magnetic layer. Ms can be further degraded by interaction with the protective overcoat. To compensate, thicker magnetic layers are required in order to provide sufficient signal. Addition of Cr and B to the magnetic layer also generally reduces the magnetic anisotropy, Ku, requiring thicker films for thermal stability.
One prior art solution is to deposit a high Ms top layer of pure Co (Ms˜1700 emu/cc) upon the CoCrPtB magnetic layer, to significantly increase signal. Unfortunately, increasing top layer thickness rapidly increases exchange coupling, with a corresponding increase in media noise.
There exists a continuing need for high areal density longitudinal magnetic recording media exhibiting high Hcr and high SMNR while overcoming the deficiencies of the prior art solutions.